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Bristow et al. (2016) 3 Off the coast of Peru and northern Chile <10 Anammox activity 16S rRNA gene and partial genes sequencing, ¹⁵N labeling incubations, FISH Anammox‐related 16S ribosomal ribonucleic acid gene sequences Hamersley et al. (2007) 4 Eastern Tropical South Pacific (ETSP) <10 Anammox activity, dissimilatory reduction of nitrate to ammonia (DNRA) Functional gene analysis, metagenome, ¹⁵N labelling incubations Candidatus Scalindua sp. T23 Lam et al. (2009), Ulloa et al. (2012) 5 Eastern South Pacific off northern Chile <10 Aerobic ammonium oxidation 16S rRNA gene and partial genes sequencing, Functional gene analysis Nitrosospira‐like βAOB Molina et al. (2007) 6 Eastern tropical North Pacific Ocean <1 Ammonia and Nitrite oxidation, DNRA, Anammox activity 16S rRNA gene and partial genes sequencing, ¹⁵N labeling incubations, Functional gene analysis Nitrospina, archaeal and β‐proteobacterial groups Beman et al. (2013), Peng et al. (2015), Pajares et al. (2019) 7 Namibian sea region <10 Nitrite oxidation FISH, ¹⁵N labelling incubations Nitrococcus, Nitrospina Füssel et al. (2012) 8 Arabian Sea OMZ <2 Denitrification, ammonia‐oxidation, anammox activity 16S rRNA gene and partial genes sequencing, N tracer incubations, Functional gene analysis Ammonia‐oxidizing archaea (AOA) and anaerobic ammonia‐oxidizing (anammox) bacteria Ward et al. (2009), Bulow et al. (2010), Pitcher et al. (2011) Sulfur metabolism 9 Northern Chilean coast <13 Sulfide oxidation, Sulfate reduction Metagenome, Functional gene analysis SUP05 group, ARTIC96BD lineage of the gamma‐proteobacteria, Desulfatibacillum,Desulfobacterium,Desulfococcus, Syntrophobacter, and Desulfovibrio Canfield et al. (2010), Crowe et al. (2018) 10 Eastern Tropical South Pacific (ETSP) <10 Sulfide oxidation 16S rRNA gene and partial genes sequencing, metagenomics and genome binning CARD‐FISH, ¹⁵N labeling incubations SUP05,Candidatus Thioglobus autotrophicus Schunck et al. (2013), Callbeck et al. (2018) 11 North eastern subarctic Pacific (NESAP) <10 Sulfide oxidation Metagenome, Functional gene analysis, genome binning Marinimicrobia clades Hawley et al. (2017) Methane metabolism 12 Eastern tropical North Pacific Ocean <10 Anaerobic methane oxidation 16S rRNA gene and partial genes sequencing, Metagenome, Functional gene analysis, genome binning, ³H‐CH₄ and ¹⁴C‐CH₄ labeling experiment NC10 bacterial clade, clade OPU3 Padilla et al. (2016, 2017), Chronopoulou et al. (2017), Thamdrup et al. (2019)

      CARD‐FISH, catalyzed reporter deposition–fluorescence in situ hybridization; βAOB, Betaproteobacteria ammonia–oxidizing archaea; DNRA, dissimilatory nitrate reduction to ammonia.

      Another study, based on the Chilean OMZ water column, also revealed the existence of active sulfide oxidizing bacterial community (SUP05/ARTIC96BD lineage of the gammaproteobacteria) that have a high affinity for sulfide and have the ability to oxidize it at very low concentration (<100 nM) (see Table 1.2 for detailed information; Crowe et al., 2018). This phenomenon indicates that such an anaerobic sulfide oxidizing bacterial community (having high affinity for sulfide) is likely to maintain vanishingly low sulfide concentrations in OMZs water‐column, thereby keeping the S cycling cryptic (rapid oxidation of sulfide into sulfate). Furthermore, another geomicrobiological exploration of ETSP region in sulfide‐poor offshore OMZ waters off the coast of Peru revealed the abundance and activity of sulfide oxidizing–denitrifying bacteria (SUP05, Candidatus Thioglobus autotrophicus, belonging to SUP05 bacterial clade) having a role in driving the S cycling; via the continued oxidation of co‐transported elemental sulfur (Schunck et al., 2013; Callbeck et al., 2018). Considering the present state of information, there was no apparent reason to presume that such sulfide‐based cryptic S cycling would be operational in geographically distinct, nitrite and sulfide rich OMZ water columns with similar geomicrobiological features. In the light of the present topic, notably, a recent study based on two ~3 m long sediment cores (collected from 530 m and 580 m below sea level) located within the pOMZ of the EAS off the west coast of India revealed active tetrathionate‐based biogeochemical S cycling in anoxic marine sediment horizon. It highlighted the role of microbial redox metabolisms in preempting the accumulation of this highly reactive polythionate in sediment pore‐fluids, over and above the known abiotic mechanisms of tetrathionate scavenging by in situ sulfide (Mandal et al., 2020). Notably, in‐depth genome data of SUP05 also revealed that this organism has the genetic potential for the utilization of thiosulfate. Thus, althoughcurrent knowledge highlights the role of sulfide, the role of other intermediate reactive redox species of S cannot be ruled out in the S cycle of the OMZ water column.

1.9. MICROBIOLOGY OF METHANE CYCLING IN THE OXYGEN MINIMUM ZONE WATER COLUMN

      High‐resolution water‐column studies revealed the existence of a 300 m thick layer with elevated methane concentrations (20–105 nM) in the anoxic core of ETNP (Chronopoulou et al., 2017). Another geochemical investigation also revealed presence of CH₄ in the eastern ETNP water column (Pack et al., 2015) (for concentrations and its oxidation rates see Table 1.2). Several omics‐based investigations revealed the presence of genes (particulate methane monooxygenase: pMMO) and transcripts (16S rRNA and other relevant functional genes) belonging to a unique group of denitrifying methanotrophs in the candidate bacterial division NC10 from Eastern Pacific OMZs

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